Balloon angioplasty is a medical procedure to widen obstructed blood vessels narrowed by plaque deposits. The procedure may be used in coronary or peripheral arteries. In an angioplasty procedure a catheter having a special inflatable balloon on its distal end is navigated through the patient's arteries and is advanced through the artery to be treated to position the balloon within the narrowed region (stenosis). The region of the stenosis is expanded by inflating the balloon under pressure to forcibly widen the artery. After the artery has been widened, the balloon is deflated and the catheter is removed from the patient.
A significant difficulty associated with balloon angioplasty is that in a significant number of cases the artery may again become obstructed in the same region where the balloon angioplasty had been performed. The repeat obstruction may be immediate (abrupt reclosure) which is usually caused by an intimal flap or a segment of plaque or plaque-laden tissue that loosens or breaks free as a result of the damage done to the arterial wall during the balloon angioplasty. Such abrupt reclosure may block the artery requiring emergency surgery which, if not performed immediately, may result in a myocardial infarction and, possibly, death. This risk also necessitates the presence of a surgical team ready to perform such emergency surgery when performing balloon angioplasty procedures. More commonly, a restenosis may occur at a later time, for example, two or more months after the angioplasty for reasons not fully understood and which may require repeat balloon angioplasty or bypass surgery. When such longer term restenosis occurs, it usually is more similar to the original stenosis, that is, it is in the form of cell proliferation and renewed plaque deposition in and on the arterial wall.
Various types of endovascular stents have been proposed and used as a means for preventing restenosis. The stents typically are tubular devices capable of maintaining the lumen of the artery open. The stent is placed inside the blood vessel after the angioplasty has been performed. A catheter typically is used to deliver the stent to the treated arterial region.
A number of patents have issued pertaining to various types of endovascular stents. Kreamer (U.S. Pat. No. 4,740,207 issued Apr. 26, 1988) discloses a radially expandable tubular stent having a latch mechanism by which a free longitudinal edge of the stent engages a longitudinally extending retaining ledge. The retaining ledge protrudes inwardly from the inner surface of the stent adjacent the opposite, free longitudinal edge of the stent. The stent has several disadvantages. First, the retaining ledge of the latching mechanism protrudes into the lumen of the stent, causing at least some obstruction to blood flow through the stent. Second, the bulk of the latching mechanism increases the profile (effective diameter) of the device when in the radially collapsed configuration. It is important for the collapsed stent to have a minimal collapsed profile allowing for easier advancement of the stent through tortuous or small diameter arteries. Third, that type of stent must be molded from a polymer in a procedure such as injection or compression molding, both of which have severe restrictions.
Such stents formed by injection molding may have wall thicknesses of the order of 0.010 inches to about 0.012 inches. Such wall thicknesses may occupy a significant portion of the lumen of the artery thereby causing potential difficulties in placement of the stent as well as potential reduction in the flow area through the artery. Additionally, such injection molded stents may require a reduction in the molecular weight of the polymer from which the stent is formed which may enhance the brittleness of the stent. Further, the injection molding process may cause warping or distortion of the stent.
U.S. Pat. Nos. 4,733,665; 4,739,762 and 4,776,337 to Palmaz disclose another type of expandable intraluminal vascular stent which, in an unexpanded configuration, can be mounted over the balloon on the distal end of a dilatation catheter. Expansion of the balloon causes a corresponding expansion of the stent. The stent is formed from metal and constructed to deform plastically and yieldably so that the stent will remain in its permanently deformed state after removal of the balloon.
European Patent Application 382,014 (Sigwart) discloses an intravascular stent in the form of a lattice-like member rolled into a small diameter spiral around the balloon of a catheter. It is retained in its small diameter configuration by a holding wire that is woven through the openings in the lattice and which can be pulled to release the rolled lattice so that it can be radially expanded by inflation of the balloon. One edge of the rolled-up lattice is provided with a plurality of outwardly projecting holding flaps that engage the lattice to prevent the device from collapsing after expansion.
U.S. Pat. No. 4,655,771 (Walsten) discloses a stent formed from a tubular braided mesh that is placed in the blood vessel in an elongated contracted configuration and self-expands when released to a larger diameter.
U.S. Pat. No. 4,649,922 (Wiktor) discloses a helical spring stent for implantation in an artery in which the stent is inserted in a contracted configuration. When released in the artery, the metallic stent springs open to its expanded larger diameter.
German Patent Application Publication No. 3640745 published Jun. 4, 1987 (Strecker) discloses a stent that can be locked in variable diameters in several successive steps.
Although many proposals have been made and considerable research is believed to have been undertaken to develop a satisfactory endovascular stent usable as an adjunct to angioplasty, no effective, satisfactory and generally accepted endovascular stent is believed to have been developed. Accordingly, there remains a need for such an effective device.